Currently, hard disk drives (hereinbelow, referred to as HDDs) rotate magnetic disks and drive head gimbal assemblies (hereinbelow, referred to as HGAs) at high speed in response to requests for huge capacity, high recording density, and high-speed accessing. These cause fluctuation of air (turbulence) to buffet the magnetic disks and HGAs. This turbulence buffeting can be a big disturbance for positioning heads for data which are recorded with high-density on a magnetic disk. This is because the turbulence occurs at random and it is difficult to estimate its magnitude and cycle so that swift and accurate positioning control will be complex and difficult. Also the turbulence buffeting may cause a noise to impair the quietness of the device.
Another problem caused by influence of the air within the device due to the high-speed rotation other than the foregoing is increase of electric power consumption. When the magnetic disk is rotated at high-speed, the air around the disk is dragged and rotated together. On the other hand, the air apart from the magnetic disk remains still so that shearing force arises therebetween to become a load against the rotation of the disk. This is called a windage loss, which becomes larger as the disk rotates at higher speed. In order to rotate the disk at high-speed against the windage loss, a motor will require a larger output and electric power.
Focusing on that the above-described turbulence and windage loss are proportional to the density of the gas within the device, there is an idea to reduce the turbulence and windage loss by enclosing low-density gas instead of air in a hermetically-sealed HDD. Hydrogen, helium, or the like is exemplified as the low-density gas, but helium is optimum because it is effective, stable, and safe in considering actual use. HDDs with sealed helium gas can solve the above-described problems and realize swift and accurate positioning control, electric power saving, and satisfactory quietness.
However, molecules of helium are so small and a diffusion coefficient of helium is large. Therefore, there has been a problem that enclosures used for usual HDDs are poorly sealed so that helium gas leaks easily during normal use. In order to make it possible to hermetically seal low-density gas such as helium gas, for example, a technique disclosed in the Japanese Patent Publication No. 2005-0068666 (“Patent Document 1”) described below has been suggested.
This Patent Document 1 discloses a magnetic disk device in which a feedthrough for coupling an FPC assembly inside the enclosure to a circuit board outside the enclosure is attached to an opening of the base and the enclosure is hermetically-sealed with a cover. With respect to the joint section of the base and the cover through which the helium inside the enclosure is likely to be leaked, the aluminum die-casted base and the aluminum cover are laser-welded so as to securely seal the joint section. With respect to the attaching section of the feedthrough through which the helium inside the enclosure is likely to be leaked, the feedthrough is constituted by a flange and a plurality of pins fixed to the flange with a sealing material such as glass or the like, and the flange is soldered to the rim of the opening on the bottom surface of the base so as to securely seal the attaching section.
As described above, the pins of the feedthrough are fixed to the flange with the sealing material. As the sealing material, glass is frequently used. The thermal expansion coefficient of the glass is significantly different from the one of aluminum used for the base. In order to avoid break of the sealing material due to thermal stress, the thermal expansion coefficient of the flange is preferably between the ones of the sealing material and the base.
Since the thermal expansion coefficient of the flange is different from the one of the base, large thermal stress is applied to the solder joint section of the flange and the base. On the other hand, the solder used for joining the feedthrough and the base has small material yield strength. For this reason, it is important to secure sufficient solder joint reliability between the feedthrough and the base in response to the thermal stress accompanying changes in temperature environment in use of an HDD. Furthermore, in solder-joining the feedthrough and the base, it is important to consider a problem that the overflowed solder contacts the pins to cause a short, and the like.